The present invention relates to image enhancement, and more particularly, to a method and circuit for enhancing the quality of an image by compensating a color signal in response to a luminance variation or adjustment due to mean separate histogram equalization and quantized mean separate histogram equalization.
The basic operation of histogram equalization is to transform a given input image on the basis of the histogram of the input image. Here, the histogram denotes a gray level distribution of a given input image. The histogram of a gray level provides an overall depiction of the appearance of an image. The gray level, appropriately controlled according to the sample distribution of an image, enhances the appearance or contrast thereof.
Histogram equalization uses a sample distribution of the image and is a widely-known contrast enhancement method. Histogram equalization has been disclosed in the following documents: [1] J. S. Lim, "Two-Dimensional Signal and Image Processing," Prentice Hall, Englewood Cliffs, N.J., 1990; and [2] R. C. Gonzalez and P. Wints, "Digital Image Processing," Addison-Wesley, Reading, Mass., 1977.
The well-known characteristics of histogram equalization becomes a defect in some actual cases. Since the output density of histogram equalization is constant, the mean brightness of an output image approaches a middle gray level. In practice, the mean brightness of the output image in histogram equalization is exactly the middle gray level regardless of the mean brightness of an input image. Obviously, this characteristics is not desirable for actual applications. For instance, a scene photographed at night appears extremely bright after histogram equalization.
Additionally, if color compensation is not performed on a color signal according to a luminance variation occurring when the above-described histogram equalization is performed, a pure color signal is distorted.
For example, consider that a color system is composed of Y, R-Y and B-Y signals, and assume that the Y signal is varied to a Y'(=Y+.DELTA.y) signal by a predetermined luminance processing technique. The varied color signal (Y',R-Y,B-Y) is varied to a value (R,G,B) without color compensation, and the resultant color signals are expressed by Equations (1) through (3): ##EQU1##
When Y is varied to Y', an exemplary pure red signal (R,0,0) is mapped to a signal (R+.DELTA.y,.DELTA.y,.DELTA.y) if there is no color compensation. Thus, the resultant color signal is no longer a pure red signal. Similarly, if there is no color compensation, all of the other pure color signals are also distorted.
Meanwhile, a contrast correction method, in which Y is extracted from input color signals R, G and B, and a correction signal is extracted from the extracted Y and added to each of the input signals R, G and B as shown in Equations (1) through (3), has been disclosed in the U.S. Pat. No. 5,345,277. However, this correction method could not solve the problem of distortion of a pure color signal.